The word sardonic used to mean what we now use sarcastic for — verbally ironic. Sarcasm comes from the Greek “to tear flesh, bite the lip in rage, sneer” and meant “bitterly cutting or caustic” when it first entered English. For me, although I understand that hypothetically you could have sarcasm that doesn’t have this inherently negative bent to it, the word still retains a fair bit of its original connotation for good reason.